U.S. patent application number 11/005823 was filed with the patent office on 2006-03-16 for seamless channel change in a wireless local area network.
This patent application is currently assigned to InterDigital Technology Corporation. Invention is credited to Christopher Cave, Angelo Cuffaro, Paul Marinier, Vincent Roy.
Application Number | 20060056344 11/005823 |
Document ID | / |
Family ID | 36033796 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060056344 |
Kind Code |
A1 |
Roy; Vincent ; et
al. |
March 16, 2006 |
Seamless channel change in a wireless local area network
Abstract
A method for seamless channel change in a wireless local area
network having a station (STA) and an access point (AP) begins by
sending a channel change intention message from the AP to the STA.
A channel change response message is sent from the STA to the AP,
informing the AP whether the STA will follow the channel change.
The AP determines whether to proceed with the channel change, and
performs the channel change if the determination is positive.
Inventors: |
Roy; Vincent; (Montreal,
CA) ; Marinier; Paul; (Brossard, CA) ; Cave;
Christopher; (Candiac, CA) ; Cuffaro; Angelo;
(Laval, CA) |
Correspondence
Address: |
VOLPE AND KOENIG, P.C.;DEPT. ICC
UNITED PLAZA, SUITE 1600
30 SOUTH 17TH STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
InterDigital Technology
Corporation
Wilmington
DE
|
Family ID: |
36033796 |
Appl. No.: |
11/005823 |
Filed: |
December 7, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60608769 |
Sep 10, 2004 |
|
|
|
Current U.S.
Class: |
370/329 ;
370/338 |
Current CPC
Class: |
H04W 36/30 20130101;
H04W 36/20 20130101; H04W 84/12 20130101; H04W 36/06 20130101 |
Class at
Publication: |
370/329 ;
370/338 |
International
Class: |
H04Q 7/00 20060101
H04Q007/00 |
Claims
1. A method for seamless channel change in a wireless local area
network (WLAN) having a station (STA) and an access point (AP), the
method comprising the steps of: sending a channel change intention
message from the AP to the STA; sending a channel change response
message from the STA to the AP, informing the AP whether the STA
will follow the channel change; determining at the AP whether to
proceed with the channel change; and performing a channel change if
the determination is positive.
2. The method according to claim 1, wherein the channel change
response message includes an acknowledgement that the channel
change intention message was received.
3. The method according to claim 1, wherein the performing step
includes: sending a channel change notification message from the AP
to the STA; changing the channel at the AP and at the STA; and
sending a channel change executed message from the STA to the AP
upon successful execution of the channel change.
4. The method according to claim 1, wherein the WLAN includes more
than one STA and the performing step considers the number of STAs
that can change channels prior to executing the channel change.
5. A method for seamless channel change in a wireless local area
network (WLAN) having a station (STA) and an access point (AP), the
method comprising the steps of: sending a channel change request
message from the STA to the AP; determining whether the channel
change can be performed; and performing the channel change based on
the determination.
6. The method according to claim 5, wherein the WLAN includes only
one STA and the determining step includes: deciding whether the
channel change will be performed by the AP; sending a channel
change notification message from the AP to the STA if the channel
change is to be performed; and changing the channel at the AP and
at the STA.
7. The method according to claim 6, further comprising the step of:
sending a channel change executed message from the STA to the AP if
the channel change has been successfully completed.
8. The method according to claim 5, wherein the WLAN includes more
than one STA and the determining step includes sending a channel
change intention message from the AP to each STA; deciding whether
the channel change can be supported by each STA; sending a channel
change response message from each STA to the AP; deciding whether
the channel change will be performed by the AP; sending a channel
change notification message from the AP to each STA if the channel
change is to be performed; and changing the channel at the AP and
at each STA.
9. The method according to claim 8, further comprising the step of:
sending a channel change executed message from each STA to the AP
if the channel change has been successfully completed.
10. A system for performing a seamless channel change in a wireless
local area network (WLAN), comprising: an access point (AP); a
station (STA); said AP configured to send a channel change
intention message to said STA; said STA configured to send a
channel change response message to said AP, said channel change
response message including an indication whether said STA will
follow said AP to a new channel; and said AP configured to
determine whether to change its channel after receiving said
channel change response message and to change its channel if the
determination is positive.
11. The system according to claim 10, wherein said channel change
response message includes an acknowledgement that said channel
change intention message was received by said STA.
12. The system according to claim 10, wherein if said AP determines
to change its channel, then: said AP is further configured to send
a channel change notification message to said STA and to change its
channel; and said STA is further configured to change its channel
and to send a channel change executed message to said AP upon
successful execution of the channel change.
13. The system according to claim 10, wherein the WLAN includes
more than one STA and said AP considers the number of STAs that can
change channels prior to executing the channel change.
14. A system for performing a seamless channel change in a wireless
local area network (WLAN), comprising: an access point (AP); a
station (STA); said STA configured to send a channel change request
message to said AP; and said AP configured to determine whether the
channel change can be performed and to perform the channel change
based on the determination.
15. The system according to claim 14, wherein the WLAN includes
only one STA; said AP sends a channel change notification message
to said STA if the channel change is to be performed; said AP is
configured to change its channel; and said STA is configured to
change its channel.
16. The system according to claim 15, wherein said STA is further
configured to send a channel change executed message to said AP if
the channel change has been successfully completed.
17. The system according claim 14, wherein the WLAN includes more
than one STA; said AP is configured to send a channel change
intention message to each STA; each STA is configured to determine
whether the channel change can be supported and to send a channel
change response message to said AP; said AP is configured to
determine whether the channel change will be performed, to send a
channel change notification message to each STA if the channel
change will be performed, and to change its channel; and each STA
is configured to change its channel.
18. The system according to claim 17, wherein each STA is further
configured to send a channel change executed message to said AP if
the channel change has been successfully completed.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/608,769, filed Sep. 10, 2004, which is
incorporated by reference as if fully set forth herein.
FIELD OF INVENTION
[0002] The present invention generally relates to a wireless local
area network (WLAN), and more particularly relates to a method for
seamless channel change in a WLAN.
BACKGROUND
[0003] In a WLAN system operating in infrastructure mode, an access
point (AP) associated to a basic service set (BSS) typically has
one or more stations (STAs) associated with it. The AP and its
associated STAs communicate with each other by sending and
receiving packets over a given channel. When this channel is
congested (i.e., the offered traffic on the channel is high), it
can be desirable for the nodes of the BSS to use a less-congested
channel. Similarly, if the channel is affected by interference
external to the system, which is not uncommon in unlicensed bands,
it may also be desirable and even necessary for the nodes of the
BSS to use a different channel.
[0004] The typical way by which BSSs change channels is for the AP
to change the channel on which it transmits all of its frames
(management, control, and data) at a specific instant. This means
that a STA would suddenly stop receiving any frames from its AP and
would see all the frames it sends to the AP remain unacknowledged.
For Quality-of-Service (QoS)-sensitive services, this service
discontinuity translates into unsatisfied users.
[0005] An amendment (IEEE 802.11h) was made to the WLAN Medium
Access Control (MAC) and Physical Layer (PHY) specifications in
order to satisfy regulatory requirements for operation in the 5 GHz
band in Europe. The amendment is meant to allow WLAN systems in the
5 GHz band to co-exist with radar systems, but it does not provide
the means by which channel changes can be performed in a manner
that is seamless to the end-users and that ensures efficient use of
the radio resources. The family of 802.11 standards does not
provide any form of hand-shaking that ensures a seamless channel
change when the AP or the STA decides to change its channel.
[0006] For a system operating in infrastructure mode, the 802.11h
specification includes a notification message allowing the AP to
notify the STA that it will be switching channels in the future.
Three options allow the notification to qualify, at a high level,
when the channel switch will occur. It is noted that the prior art
does not provide any mechanisms for the following:
[0007] 1) The STA to request the AP to change the channel.
[0008] 2) The STA to acknowledge that it has received the
notification from the AP and that it will follow the AP to the new
channel. In 802.11h, the notification is performed using a
broadcast, so the AP does not receive any acknowledgement (ACK)
allowing it to determine if the STA will follow it to the new
channel or if the STA correctly received the notification. This can
create a situation where an AP needlessly transmits and retransmits
frames to a STA that did not follow the AP to the new channel. The
number of frames buffered in the AP can be substantial and could
lead to a dramatic inefficiency of the wireless medium due to the
exponentially increasing back off window associated with multiple
retransmissions.
[0009] 3) The STA to acknowledge that it has followed the AP to the
new channel. This can create a similar inefficiency situation as
described above.
[0010] Accordingly, a need exists for a method and apparatus for
seamless channel change in a WLAN.
SUMMARY
[0011] The present invention addresses the limitations of the prior
art, thus allowing a seamless channel change in a WLAN without
service interruption and without dramatic reduction of the
efficiency of the wireless medium.
[0012] A method for seamless channel change in a WLAN having a STA
and an AP begins by sending a channel change intention message from
the AP to the STA. A channel change response message is sent from
the STA to the AP, informing the AP whether or not the STA will
follow the channel change. The AP determines whether to proceed
with the channel change, and performs the channel change if the
determination is positive.
[0013] Another method for seamless channel change in a WLAN having
a STA and an AP begins by sending a channel change request message
from the STA to the AP. A determination is made whether the channel
change can be performed, and the channel change is performed based
on the determination.
[0014] An optional additional step for both of these methods
includes sending a channel change executed message from the STA to
the AP upon successful execution of the channel change.
[0015] A system for performing a seamless channel change in a
wireless local area network includes an access point (AP) and a
station (STA). The AP is configured to send a channel change
intention message to the STA. The STA is configured to send a
channel change response message to the AP, including an indication
whether the STA will follow the AP to a new channel. After
receiving the channel change response message, the AP is further
configured to determine whether or not it will proceed with the
channel change and to perform the channel change if the
determination is positive. Although the AP will consider the
response message from the STA, the decision whether to proceed with
the channel change does not need to be in accordance with the
response message.
[0016] Another embodiment of a system for performing a seamless
channel change in a wireless local area network (WLAN) includes an
access point (AP) and a station (STA). The STA is configured to
send a channel change request message to the AP. In the case where
the AP only supports one STA, the AP is configured to determine
whether the channel change can be performed, and to perform the
channel change based on the determination. In the case where the AP
supports more than one STA, the AP is configured to initiate the
channel change procedure via the channel change intention message,
as described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] A more detailed understanding of the invention may be had
from the following description of a preferred embodiment, given by
way of example, and to be understood in conjunction with the
accompanying drawings, wherein:
[0018] FIG. 1 is a flowchart of a method for seamless channel
change initiated by an AP in accordance with the present
invention;
[0019] FIG. 2 is a signal flow diagram of the method shown in FIG.
1; and
[0020] FIG. 3 is a flowchart of a method for seamless channel
change initiated by a STA in accordance with the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] Hereafter, the term "station" (STA) includes, but is not
limited to, a wireless transmit/receive unit, a user equipment, a
fixed or mobile subscriber unit, a pager, or any other type of
device capable of operating in a wireless environment. When
referred to hereafter, the term "access point" (AP) includes, but
is not limited to, a base station, a Node B, a site controller, or
any other type of interfacing device in a wireless environment.
[0022] The present invention solves the problem described above by
setting up a hand-shaking procedure between an AP and a STA in
which the AP communicates to its associated STAs its intention to
switch channels and by which the STA acknowledges that it will or
will not follow the AP on the new channel. This procedure ensures
that all STAs are aware of the AP's intention to change the channel
and that the AP is aware of which STAs are able to follow to the
new channel and which STAs would have followed to the new channel
had the AP decided to go forward with the channel change. The
present invention also solves the problem by allowing the STA to
request a channel change, thus triggering the hand-shaking
procedure.
[0023] The AP initiated hand-shaking procedure 100 of the present
invention is illustrated in FIG. 1. For purposes of simplifying the
drawings and the discussion of the invention, the following
description only refers to a single STA. The principles of the
present invention can be equally applied to a system which has
multiple STAs.
[0024] An AP sends a STA a channel change intention message
indicating its intention to change its channel to channel X, where
X represents a channel identifier (step 102). The message also
contains information relative to the timing of the channel change.
This message can be sent using a broadcast frame or a unicast
frame. The advantage of using a broadcast frame is that it limits
the number of messages sent over the wireless medium when multiple
STAs are associated to the AP. The advantage of using a unicast
frame (one for each associated STA) is that it tends to increase
the robustness of the signaling, since the AP expects a MAC ACK
from the STA indicating whether or not the STA has correctly
received the message. In the case where no ACK is received from a
certain STA, the AP can re-send the channel change intention
message.
[0025] Upon reception of the channel change intention message, the
STA determines if it will change its channel to the new channel
based on its capabilities, the radio frequency environment it
perceives from its location, and the availability and load of other
APs in the system (step 104). This determination involves a
multiple criteria decision-making process. For example, the first
criterion the STA could consider is whether it has the capabilities
(considering hardware and configuration limitations) to change to
channel X, as announced in the channel change intention message
sent by the AP. The second and third criteria the STA is likely to
consider include the level of interference and load the STA
perceives on channel X. Based on such criteria, the STA decides
whether the channel change is desirable or not. This conclusion can
be reached using any form of multiple criteria analysis that
compares scores obtained from a weight-based calculation against
predetermined levels.
[0026] Once the decision is made, the STA sends a channel change
response message which includes a notification that the message was
received (applicable in the case where the channel change intention
message is sent using a broadcast), and an indication as to whether
or not the STA will follow the AP to its new channel (step
106).
[0027] The indication in the channel change response message
contains various pre-defined responses including, but not limited
to:
[0028] 1) The STA will follow the AP to the new channel.
[0029] 2) The STA will not follow to the new channel and would like
to continue to be served by the AP on the same channel. This
situation could arise if the STA's scanning indicates that the new
channel will degrade its performance, if the STA lacks the
capabilities to change channels, or if the STA determines that
changing the channel will not allow it to satisfy the QoS
requirements of the user's application.
[0030] 3) The STA will not follow the AP to the new channel but
does not request the AP to stay on the same channel. This situation
could arise if the STA has identified another candidate AP which it
determines can offer better performance than the current AP on the
new channel.
[0031] The range of techniques by which a STA could perform the
channel change determination include using measured or reported
quality metrics (e.g., signal to noise ratio (SNR), signal to
interference and noise ratio (SINR), packet error rate (PER), etc.)
on the new channel and comparing these metrics to quality metrics
measured on the current channel or to predetermined levels. The STA
could also use measured or reported load metrics (e.g., channel
occupancy, medium access delay, etc.) on the new channel and
compare them to load metrics measured on the current channel or to
predetermined levels. It is also possible to use weight-based
combinations of several of the above metrics obtained on the new
channel, to be compared against the same metrics obtained on the
current channel or against predetermined levels.
[0032] If the AP sends its channel change intention message using a
broadcast, the channel change response message sent by the STA will
also serve the purpose of acknowledging the reception of the
channel change intention message. There are some advantages in
sending the channel change intention message as a unicast message
(which commands for an ACK), since it allows the AP to discriminate
between a STA that did not receive the channel change intention
message and a STA that received the channel change intention
message but did not send the channel change response message. The
AP waits for all responses from all STAs (positive or negative) or
until a timeout occurs, and then makes the decision whether it will
go forward or not with the channel change. The AP is not bound to
do what the STA suggested.
[0033] Based on the channel change response notification received
from the STA, the AP then makes a decision whether it is going to
change the channel or not (step 108). This step allows the AP to
reconsider its intention to change the channel. For example, in the
case where the AP only serves a single STA and this STA indicates
that it cannot follow the AP to the new channel, the AP might
decide not to perform the channel change. This is also an
opportunity for the AP to request measurements from the STAs if it
believes that measurement reports will help it to make a better
decision.
[0034] In general, the determination of whether or not the AP will
go forward with the channel change is based on the relative number
of users that have responded with each type of channel change
message versus the foreseen impact on quality of service of not
performing the channel change. A decision making process which
compares the relative number of responses of each type against
targets that vary according to the foreseen performance gain of
switching the channel is one of several potential
implementations.
[0035] If the AP decides not to change channels, the procedure 100
terminates (step 110). In the event that the AP decides to go
forward with the channel change (step 108), it then sends a channel
change notification message to the associated STA (step 112). The
message also contains information relative to the timing of the
channel change. This message can be sent using a broadcast frame or
a unicast frame, with the advantages of each type of frame being
the same as described above.
[0036] If the AP decides to go forward with the channel change, it
can use the information contained in the channel change response
message received from the STA and remove all the buffered packets
dedicated to the STA that responded that it would not follow the AP
to the new channel. This prevents the AP from wasting a
considerable amount of bandwidth by unsuccessfully transmitting
packets to the STA if it has not followed the AP to the new
channel. The AP changes to the new channel (step 114) and the STA
changes to the new channel if it is following the AP (step
116).
[0037] An optional step for the hand-shaking procedure is to have
the STA send the AP a channel change executed message after it has
changed the channel (step 118). This information could also be used
to prevent the AP from wasting bandwidth by unsuccessfully
transmitting packets to a STA that would have indicated (in its
channel change response message) that it would follow the AP to the
new channel but that would have failed to do so. The procedure then
terminates (step 110).
[0038] FIG. 2 is a signal diagram of a system 200 executing the
procedure shown in FIG. 1. The system 200 includes an AP 202 and a
STA 204. The AP 202 sends a channel change intention message 210,
which can be sent as a broadcast or as a unicast. The STA 204 (or
each STA 204 if the AP 202 has multiple STAs associated to it)
responds to the intention message 210 by sending a channel change
response message 212 to the AP 202. Each STA 204 sends its response
message as a unicast message.
[0039] If the channel change is going to occur, the AP 202 sends a
channel change notification message 214 to the STA 204. As with the
intention message 210, the notification message 214 can be sent as
a broadcast or a unicast message. As an optional last message, the
STA 204 can send a channel change executed message 216 to the AP
202 upon a successful channel change.
[0040] A method 300 in accordance with the present invention by
which a STA requests the AP to change its channel is shown in FIG.
3. In a preferred embodiment, the present invention enables the STA
to request the AP to change the channel it uses. The need for such
a request may arise when the interference or channel activity
perceived by the STA is such that it jeopardizes the QoS perceived
by its user.
[0041] The STA sends a change channel request message as a unicast
frame to its AP (step 302). The message can include some or all of
the following information: time limit for performing the channel
change, list of preferred channels on which to migrate to, and
interference or noise level measurements on the current and
candidate channels.
[0042] Upon reception of this message, the AP can then take one of
two actions. If the AP has multiple STAs associated to it, it could
perform the AP initiated hand-shaking procedure 100, beginning at
step 102. If the AP only has a single STA associated with it, it
could decide to go forward with the channel change procedure, and
perform the procedure 100 beginning at step 108.
[0043] At any point in the flow of events described in FIG. 1, the
AP can take measurements on the current and proposed channels
and/or ask for measurements from the STA requesting the channel
change.
[0044] Exemplary applications of the present invention include the
areas of load balancing and dynamic channel selection. In these
applications, it is desirable for the AP to change its channel in
order to adapt to time-varying load and interference conditions.
The present invention allows for such channel changes to be
performed with minimal service interruption for associated
STAs.
[0045] Although the features and elements of the present invention
are described in the preferred embodiments in particular
combinations, each feature or element can be used alone (without
the other features and elements of the preferred embodiments) or in
various combinations with or without other features and elements of
the present invention. While specific embodiments of the present
invention have been shown and described, many modifications and
variations could be made by one skilled in the art without
departing from the scope of the invention. The above description
serves to illustrate and not limit the particular invention in any
way.
* * * * *